The invention relates to a belt retractor comprising a retractor shaft and a device for determining the rotational speed and/or the angle of rotation.
In the case of belt retractors, especially of retracting tensioners, it is necessary for particular functions to know the angle of rotation of the retractor shaft. This is necessary, for example, for detecting the out-of-position situation, for the rotational speed monitoring and the rotational speed control of the motor for driving the tensioner or a pre-tensioner or for controlling the tensioning operation itself.
Usually the angle of rotation is determined by means of an artificially generated alternating magnetic field. A magnetic wheel or any other magnetic component is connected to the retractor shaft in a rotationally fixed manner so that a periodically alternating magnetic field is generated during rotation of the turning shaft. Said field is detected by means of a suitable magnetic field sensor usually a Hall sensor, and the rotational speed and/or the angle of rotation of the retractor shaft is concluded from the information obtained therefrom. The Hall sensors are fastened directly on an ECU board so that for detecting the magnetic field the entire board has to be placed as closely as possible to the retractor shaft, which entails considerable constructional problems.